Posted by dsyer on Apr 02, 2019; 2:35pm
URL: https://discuss.asciidoctor.org/Filename-extension-changed-by-default-when-upgrading-to-asciidoctor-2-tp6822p6823.html

Update: looks like `Asciidoctor::DEFAULT_EXTENSIONS['<backend>'] = '.html'` was what I needed, where `<backend>` is the id of the backend.